1. Field of the Invention
This invention is related to the field of solid composite preparations generally and to the preparation of solid composites having self-lubricating and wear resistant characteristics in particular.
2. Prior Art
Those active in the field of tribology are forecasting a need for composite solid lubricants that are effective at temperatures up to 700.degree. F. Such forecasts are based on future requirements for self-lubricating resin composites used in bearings for nuclear reactors, gas turbine engines, space vehicles, and high performance air frames. At present, commercially available self-lubricating composites do not perform satisfactorily at temperatures above 500.degree. F (260.degree. C). Moreover, at the high end of their temperature ranges, the wear rates of these composites are relatively high and their friction erratic. This limits their lubricating performance in low torque bearings and in dynamic seal components.
The appropriate lubricant filler and the geometry and composition of the reinforcing agent are important in influencing the high temperature tribological behavior of polymeric composites, but the controlling parameters are the thermal stability and strength of the base resin under load and high temperature. For example, Giltrow and Lancaster (Giltrow, J. P., and Lancaster, J. K., "Carbon Fibers in Tribology," Soc. Chem. Ind., Third Conf. on Carbons and Graphite, London, 1970) showed that the wear rates of PTFE and nylon at room ambient temperatures were higher than that of polyphenylene oxide, although all of these were reinforced with graphite.
Following their development, certain of the thermoplastic polymers, such as polyphenylene sulfide, nylon, "Teflon," and the polyimides, have been used as base binders or matrix resins for lubricating substances (additives such as MoS.sub.2 or WS.sub.2) and reinforcing agents. Some of the base binders themselves as well as some of the reinforcements were found to have inherently good lubricating qualities. Examples of binders with these characteristics are polytetrafluoroethylene, nylon, and polyimide. Examples of reinforcements are the Type I high elastic modulus graphite fibers.
In general, it can be concluded that self-lubricative polyimide composites prepared from condensation-type polyimides are known. However, these composites exhibit lower strength and wear life than composites of the instant invention, largely because of the presence of voids attributed to the liberation of gases during the cure of these materials.
Lubricative composites prepared from thermoplastic polyimides which depend solely upon a high glass transition temperature to provide high temperature properties exhibit the disadvantage of requiring unacceptably high fabrication temperatures (.apprxeq.700.degree. F) as well as the disadvantage of deforming under load at high temperatures.
In those instances where conventional addition type polyimides, derived from bismaleimides, have been used to fabricate lubricative composites, the composites were found to be limited in their use to temperatures of less than 550.degree. F and to exhibit marginal resistance to frictional heat which tends to increase the wear rates of the composites.
Applicants know of no lubricative composites which exhibit chemical, physical and thermal characteristics comparable to those of the instant invention.